ABSTRACT Mitochondria not only serve as the major source of cellular energy, but also as a coordinator of the highly sophisticated metabolic system. Coordination requires communication, and thus our long-term interest is in how mitochondria transmit messages to regulate metabolic homeostasis. Mitochondrial signaling has emerged as a key regulator of aging, but signals that have been described to date are not encoded in the mitochondrial genome. The identification of Humanin, a peptide encoded in the mitochondrial DNA, provided a paradigm-shifting regulatory mechanism of mitochondrial communication. We have recently discovered a novel peptide encoded within the mitochondrial DNA and named it MOTS-c (Mitochondrial ORF within the Twelve S rRNA). MOTS-c acts on the skeletal muscle and promotes cellular glucose and fatty acid metabolism, mediated by the folate-AMPK pathway. In mice, MOTS-c regulates glucose homeostasis and prevents obesity and insulin-resistance in high-fat fed young mice. We have also obtained evidence supporting MOTS-c-dependent regulation of metabolic aging: (i) MOTS-c levels in mice decline with age in circulation and skeletal muscle concomitantly with the development of muscle insulin-resistance and (ii) systemic injection of MOTS-c for a week sufficiently reversed age-dependent muscle insulin resistance. We hypothesize that MOTS-c is a mitochondrial-encoded regulator of the folate-AMPK pathway that promotes metabolic homeostasis and that restoring the age-dependent decline of MOTS-c can reverse metabolic aging. We propose to study (i) the impact of aging on MOTS-c biology and conversely (ii) the effect of MOTS- c on aging metabolism. We will take a top-down approach with 3 aims to test our hypothesis. Aim 1 will determine the age-dependent impact of MOTS-c on metabolic aging in mice. Aim 2 will examine the role of MOTS-c in regulating cellular metabolism in young vs aged primary muscle cells. Aim 3 will test the folate- AMPK pathway in mediating MOTS-c-dependent metabolism during aging. These findings will add an entirely novel ?mitochondrial-centric? mechanistic layer to the regulation of aging metabolism, and provide a new therapeutic target for age-dependent metabolic conditions.